A wide range of reactions that occur in aqueous solution are heavily dependent on the concentrations of available protons or hydroxide ions. The ability to control solution pH, where pH=−log [H3O+], is required for many reactions in biochemistry, synthesis and processing, and colloid chemistry, as well as for a myriad of technological processes that are performed in water. As many of the reactions of interest consume or release either protons or hydroxide ions, reagents that can buffer the solution pH to specific values are often required. When a given process or reaction only requires a single pH value, standard pH buffers are adequate. However, some processes require the ability to switch from one pH to another. One example of such a process is precipitation, where a homogeneous pH change is required to move from a regime in which precursors are soluble into a regime where the solution is supersaturated and nucleation and growth can occur. See B. C. Bunker et al., Science 264 (5155), 48 (1994).
A specific example in which reversible and programmed pH changes can be a powerful tool involves the reversible sequestration of CO2 from the atmosphere. The U.S. Department of Energy has set a goal for removing over one billion metric tons per year of CO2 from air and/or the effluent of coal fired power plants. One inexpensive mechanism for achieving this goal is to program water to reversibly capture and release the CO2. As shown in FIG. 1, capture can be achieved at high pH by converting relatively insoluble CO2 into the highly soluble anionic bicarbonate ion (up to a theoretical limit of 11 M). See D. A. Palmer and R. Vaneldik, Chemical Reviews 83 (6), 651 (1983). Conversely, release is promoted by switching back to low pH, which converts the soluble bicarbonates back to insoluble CO2 gas. While such pH changes could be achieved by cycling between additions of strong acids and bases to the solution, it would be more efficient to have a programmable pH buffer in the system as a reservoir for reversibly adding or removing either protons or hydroxide ions without the need to continuously add and consume external reagents.